Means to overcome low flow problems of inducers in centrifugal pumps

ABSTRACT

A centrifugal pump having a suction inlet, a discharge outlet and at least one pumping stage with an impeller in direct communication with the suction inlet has an inducer means in the suction inlet operatively associated with the impeller of said pumping system and an annular chamber or passage formed about the suction inlet which is provided with ports or openings in communication with the suction inlet downstream of the inducer means and upstream of the inducer means to permit the automatic bypassing or recirculation of a given portion of the fluid being pumped approximately as a direct function of the differential pressure established across the inducer means by the quantity of fluid being pumped. Angled guide vane means in the portion of the bypass or recirculation chamber or passage which communicates with the upstream side of the inducer means are provided to impart prerotational movement in the recirculated fluid in a direction of rotation the same as that of the inducer means the bypass means and guide vane means acting to maintain a reduced head across the inducer particularly at low or partial flow conditions.

[ Mar. 27, 1973 United States Patent Berman [57] ABSTRACT A centrifugal pump having a suction inlet, a discharge 154] MEANS TO OVERCOME LOW FLOW PROBLEMS OF INDUCERS IN CENTRIFUGAL PUMPS outlet and at least one pumping stage with an impeller in direct communication with the suction inlet has an [75] Inventor: Merril Berman, Shawnee Mission,

Kans.

[73] Assignee: Worthington Corporation, Harrison [22] Filed: May 21, 1971 Appl. No.: 145,804

to permit the automatic bypassing or recirculation of a given portion of the fluid being pumped approximately as a direct function of the differential pressure established across the inducer means by the quantity [52] U.S.Cl........................415/53,4l5/73,4l5/l43 [51] Int. 5/00 'Fi ld of searchwnmm of fluid being pumped. Angled guide vane means in the portion of the bypass or recirculation chamber or passage which communicates with the upstream side of the inducer means are provided to impart prerotational movement in the recirculated fluid in a direction of rotation the same as that of the inducer S T N m 6 WA alP CS E m mT R e D E W N U N h.

883 10/1937 Clas0n....................................4l5/53 841 4/1943 Hagen means the bypass means and guide vane means acting 366 19 Klein et aL to maintain a reduced head across the inducer particu- 675 l/l95'8 Gilman.................. larly at low or partial flow conditions.

850 3/1965 Englesberg et al.

384 10/1969 Boone et al.

5 Claims, 12 Drawing Figures n h m G 35 b o B ..m a we we .mP mw XP i, We m mm rt PA PATENTEUHARZYIQYS SHEET 10F 3 FIG.4

FlG'S MERRIL BERMAN INVENTOR.

PATENTl-lnmzvma SHEET 2 [IF 3 FIG.5

FIG. 7

MERRIL BERMAN F G. 6 INVENTOR.

MEANS TO OVERCOME LOW FLOW PROBLEMS OF INDUCERS IN CENTRIFUGAL PUMPS BACKGROUND OF THE INVENTION This invention relates generally to centrifugal pumps having inducers more particularly multi-stage centrifugal pumps with an inducer means associated with the first pumping stage impeller.

As will be recognized'by those skilled in the art, a centrifugal pump can be designed to operate within certain head and capacity ranges. For each centrifugal pump design there will be a net positive suction head (NPSI-I) at any given set of head, capacity conditions at which the pump is being operated.

Where it is necessary to operate at lower NPSH than that for which the centrifugal pump will continue to operate without cavitation or without requiring increased horsepower requirements to drive the pump, inducers have been developed to coact generally with the impeller in the first pumping stage of the centrifugal pump by imparting rotational motion to the fluid entering the impeller eye.

One form of the impeller for this purpose comprises a hub with radially extending blades which is connected to and rotated by the shaft which drives the impeller and is located forward of the inlet eye of the-impeller in the suction inlet for the pump. The blades of this type inducer means are fixed and generally define a substantially constant angle with respect to the longitudinal or axial line of the pump.

The angle of the inducer blades generally termedth design angle will be such that at a certain critical head and capacity operating condition the angle of attack between the fluid being pumped and the blade angle of the inducer means will approach zero.

The effect at the design condition as will be understood by those skilled in the art is that the'head or differential pressure across the inducer means will be negligible. Consequently, very little power will be required to drive the inducer means and the attendant advantages of the inducer means are obtained without additional cost or expense in regard to the manufacture of a particular pump and it permits the use of smaller and more compact centrifugal pumps for given head and capacity ranges than are obtainable from centrifugal pumps without inducer means thereon.

However, at flow conditions below those for which the blade angle of the inducer is designed there will be an effective angle of attack and the inducer will produce higher heads.

Since, higher head will require higher horsepower to drive it, when the inducer is used in a centrifugal pump having a flat horsepower curve the pump will require an increase in the size of the motor if it is to be operated at reduced flows for any material period of time.

Additionally, the higher rise in pressure across the inducer is also undesirable, as it will make it necessary to use higher rated valves, piping and pressure vessels downstream of the pump. The increased costs due to increase motor sizes and to meet the problem of increased pressure capabilities are thought to be selfevident. These problems incident to reducedflow conditions can be overcome by recirculating a portion of the fluid being pumped and imparting prerotation to the recirculated fluid in such a manner that the angle of attack between the fluid and the inducer blades will be reduced at partial flow thus reducing the head across the inducer.

SUMMARY OF THE INVENTION Accordingly, the present invention covers a centrifugal pump having a suction inlet, a discharge outlet, at least one pumping stage having an impeller in direct communication with the suction inlet and an inducer means in the suction inlet associated with the impeller; self regulating bypass means for recirculating a given portion of the fluid being pumped from the discharge or downstream side of the inducer means to the inlet or upstream side of the inducer approximately as an direct function of the differential pressure across the inducer means and angled guide vanes produced in the bypass means to-impart prerotational movement to the recirculated fluid in the same direction of rotation as that of the inducer means the bypass means and guide vanes to effect a reduction in head across the inducer means particularly at the reduced flow conditions.

It is an object of the present invention to provide in combination with a centrifugal pump having an inducer means, means to reduce or substantially eliminate the requirement for increased motive power where the pump is to be used at flow conditions below design flow conditions for the pump.

It is another object of the present invention to provide in combination with a centrifugal pump having an inducer means to overcome low flow problems of the inducer particularly incentrifugal pumps having a flat horsepower curve both in terms of the motor power required and in terms of reduction in system pressure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 is a cross sectional view through the plane 3- I 3 of FIG. 2;

FIG. 4 is a partial elevational cross sectional view through the plane 4-4 of FIG. 2;

FIG. 5 is a side elevational view, partly in cross section, of a single-stage horizontal pump utilizing an inducer sectionaccording to the invention;

FIG. 6 is an elevational view, partly in cross section, through the plane 66 of FIG. 5;

FIG. 7 is a partial cross sectional view through the plane 7-7 of FIG. 5; and

FIG. 8 is a conventional head-capacity curve for an inducer in a centrifugal pump illustrating the design flow condition for the inducer thereof;

FIG. 9 is a diagrammatic inlet diagram showing the angle of attack between the fluid being pumped and the blades of an inducer at the design flow conditions for the centrifugal pump illustrated in FIG. 8;

FIG. is a diagrammatic inlet diagram showing the angle of attack between the fluid being pumped and the blades of an inducer at reduced or partial flow condition in the centrifugal pump illustrated in FIG. 8;

FIG. 11 is a diagrammatic inlet diagram showing the improvement in the angle of attack in a centrifugal pump using the present invention;

FIG. 12 is a comparative graph illustrating the effect of an inducer with by-pass and prerotation-in accordance with the present invention on the head capacity and horsepower capacity curves of a Vertical Turbine Pump.

Referring to FIG. 1 a vertical turbine pump generally designated 10 which embodies the present invention is illustrated in association with a well means 11 which may be part of the hot well of the condenser or associated with a stripping system as is shown in U. S. Pat. No. 3,276,384.

Fluid to be pumped passes or is delivered from the source (not shown) through the inlet 12 for the well means I1 and this fluid is pumped from the well means 11 by the pump 10.

Pumping arrangements of this type are well known and accordingly the pumping portion of pump 10 is generally of conventional design and comprises a plurality of casing assemblies each respectively forming and communicating with each other in the following order, a suction inlet generally designated 20, a plurality of pumping stages as at 21a and 21b and a connecting conduit 22. The connecting conduit casing 22 in turn is connected to and communicates with a housing 23 forming a collecting chamber having an outlet 24 which is connected to the point of use or storage (also not shown) where the pumped fluid will be used or stored.

The pumping stages 21a and 21b each include respectively an impeller 25a and 25b. Impellers 25a and 25b are fixedly connected to a vertically disposed shaft 26 which extends upwardly through the various casing assemblies above described to a coupling means 27 which couples the shaft 26 to a driving shaft 28 of a suitable driving means 29 generally an electric motor.

The casing assembly which forms the suction inlet generally designated 30 has an inlet opening as at 31 so as to permit fluid in the well means 11 to enter into the suction inlet 30.

Thus, when the driving means 29 is placed into operation so as to rotate the vertical shaft 26 connected thereto the impellers a and 25b will be rotated and assuming there is sufficient suction head the impeller 25a will pump fluid from the well 11 through the suction inlet and impeller 25a to the impeller 25b which in turn will move the fluid through the connecting conduit 22 into collecting chamber 23 where the collected fluid will be displaced through the discharge outlet 24 and delivered to the point of use or storage as the case may be.

This operation is known and conventional and requires no further description as it will be clearly understood by those knowledgeable in the pumping art.

Where, however, there is insufficient suction head or where the quantity of fluid to be pumped is reduced in order to establish the necessary pumping head, pump 10 is further provided with an inducer 35 which is mounted on the end of a jack shaft 36 extending along the longitudinal line of the suction inlet 30 through a bearing member 37 supported by radially extended jack shaft 36 and the inducer fixedly connected thereto will rotate simultaneously therewith.

Whenever the head conditions or the flow rate conditions are such that the net positive suction head acting in the suction inlet is below the design conditions for which the impellers of the pump 10 will operate without an inducer, the induceracts to rotate the incoming fluid to permit the impeller to operate at a lower net positive suction head and thus increases the capacity range for effective operation of pump 10.

The inducer is essentially a propeller pump with blades of constant oralmost constant angle along any streamline. At the head and flow conditions for which the inducer is designed it creates very little head and therefore takes very little horsepower to drive it. As a consequence, it will not effect theefficiency of the pump nor will it increase the horsepower required to drive the pump.

However, at flows below normal the inducer will produce higher heads as there is an effective angle of attack between the fluid flowing through the suction inlet and the fixed blades of the inducer. The higher head of the inducer increases the horsepower required to drive the pump.

Therefore, when an inducer is used on a pump having a flat horse power curve an increased motor size will be required for such pump if it is to be operated at reduced flows at any time.

Additionally, as above indicated the high rise in pressure due to the increased head of the inducer and the larger driving means will produce a corresponding high rise in pump operating pressure. This is undesirable as it will require higher rated valves, piping and pressure vessels downstream of the pump.

The object of the present invention is to achieve more economical and improved operation of inducers during operation at lower than net positive suction head or low flow design conditions. This is accomplished first by providing means to recirculate a portion of the fluid from a point in the suction inlet downstream of the inducer to a point in the suction inlet upstream of the inducer and second by prerotating the recirculated or bypassed fluid entering the suction inlet upstream of the inducer in a direction or rotation substantially similar to the direction ofrotation of the inducer itself.

The advantage of bypassing or recirculating a por-;

tion of the fluid being pumped and prerotating the same is to minimize the effect of the reducing in fluid flow above stated.

The overall result is to reduce both the system output pressure and the required prime mover force for driving the inducer that has been heretofore required by the prior art structures.

fect of the inducer on the respective head capacity and horsepower capacity curves for a conventional vertical turbine pump with just the inducer alone. The dashed line provides a comparison which shows the effect on the same pump with a bypass and prerotation means operatively associated with the inducer in accordance with the present invention.

The solid line shows that there is a marked increase in both head and horsepower requirements at reduced capacity for the reasons above set forth.

The dashed line shows that the same inducer with bypass and prerotation produces substantially less head and a markedly lower horsepower requirement at reduced heads.

Thus, utilizing the present invention I have found that at reduced flow the inducer head does not increase and the requirement for increased horsepower and other side effects of increased pressure are eliminated.

It will be further noted that the construction and arrangement of the bypass and prerotation means for the inducer above described effects these desirable and advantageous results automatically. It is self regulating as a function of the head or pressure conditions across the inducer. Thus, the bypass flow is least at design conditions and increases as the inducer head increases at reduced flow condition. The inducer head is least at design conditions or normal flow because the inducer head approaches zero. If the inducer head increases due to reduced or partial flow the higher differential pressure created across the inducer causes the gypass or recirculated flow to increase and flow across the inducer is not reduced to the extent that the flow through the pump has been reduced.

The prerotation of this bypass flow coupled with the self regulation accomplishes the desired end result.

The structure described prevents the inducer from operating at no flow conditions and maintains the flow through the inducer greater than through the pump at all reduced or partial flow conditions. It acts at all times to match the entrance flow to the inducer to the design angle for the blades of the impeller so that angle of attack between the fluid being pumped and the blade angle of the inducer will approach zero.

These conditions reduce developed head and required horsepower of the inducer as pumped through flow is reduced and the surging experienced with inducers at partial or reduced flow will also be reduced. The advantages of reducing hydraulic surging at the inducer is important as it reduces vibration and rapid wear of pump shafts and bearings.

FIG. 6 FORM OF THE INVENTION FIG. 6 illustrates a horizontal centrifugal pump generally designated 110 having an inducer and bypass and recirculation means substantially similar in construction and operation to that above described for the form of the invention shown in FIGS. 1-4 of the drawings.

Pump 110 includes a casing 111 which forms a volute or pumping chamber 112 having a discharge outlet 113. The casing has an opening 114 and a casing cover 115 which is connected to the casing 111 about the opening 114 as by circumferentially spaced threaded members 116.

The casing cover 115 has an axially extending cylindrical projection 117 thereon forming a suction inlet generally designated 118 which has an inlet opening as at 119 to permit fluid to be delivered to the suction inlet from any suitable source by the connecting conduit 120 which is connected to the casing cover 115 as is hereinafter described. I

The pump 10 will be provided with a driven shaft 121 which is mounted along the axial line of the pump by any suitable bearing means (not shown). The driven shaft has one end extending through the volute or pumping chamber 112 into the suction inlet 118 and the other end extends out of the pump to a coupling 122 which connects the driven shaft 121 to a drive shaft 123 of any suitable prime mover 124 such as an electric motor.

An impeller 12S disposed in the pumping chamber 112 is fixedly connected and rotatable with the driven shaft 121 and is disposed so that the suction eye 126 of the impeller communicates with the suction inlet 118.

Outboard of the impeller and cooperating therewith is an inducer 127 also fixedly connected to and rotatable with the driven shaft 121.

In operation when the prime mover is started and fluid is delivered to the inlet opening 119 of the suction inlet 118 the fluid is drawn into the eye 126 of impeller 125 and discharged through outlet 113 which operation will continue as long as the prime mover is operating and there is sufficient fluid at a net positive suction head for whatever the special characteristics are for a given centrifugal pump of this type.

Where the net positive suction head for the pump drops below the design conditions for the impeller, the inducer comes into operation and permits the impeller to operate at reduced heads below the normal net positive suction head for the impeller or at reduced flow conditions.

However, at reduced flow condition the problems as above described occur and accordingly the form of the invention as shown in FIGS. 6, 7 and 8 is provided to overcome the low flow problems of such inducers.

Thus, similar to the form of the invention shown in FIG. 1-4 the pump 110 is provided with a generally axially extending cylindrical member 130 about the axially extending cylindrical projection 117 forming the suction inlet 118. Cylindrical member 130 is spaced from the cylindrical projection 117 to form a recirculation chamber or bypass passage generally designated 131.

The annular member 130 is connected at one end to the casing cover 115 as by threaded member 132 and at the end remote therefrom which extends axially beyond the outer end of the axially extending projection 117, the cylindrical member 130 is provided with an inwardly extending flange 133.

The inwardly extending flange 133 forms an opening as at 134 which is equal to the size of the inlet 119 for the suction inlet 118 and the annular member extends beyond the end of the axial extending projection 117 forming the suction inlet so that there is a space therebetween and disposed in said space are a plurality of circumferentially spaced guide vanes 135.

AUTOMATIC BYPASS OR RECIRCULATION AND PREROTATION MEANS The means for accomplishing this advantageous result is easily effected as can be seen by reference to FIGS. 1-4 of the drawings wherein the suction head casing assembly forming the suction inlet is shown in cooperation with a generally cylindrical outer member 40 which is disposed about the suction inlet casing assembly 20 and spaced therefrom to define and form an annular fluid chamber or passage 41.

The upper end of the annular member 42 is connected to the upper end of the casing assembly 20 as by threaded members 42 as can be seen by reference to FIGS. 1 and 2 of the drawings.

Annular member 40 extends beyond the lower end of the suction inlet casing assembly 20 so that an inwardly. extending flange 44 formed on annular member 40 and provided with an opening as at 440 which is equal to the inlet opening 31 of the suction inlet 30 will be disposed a spaced distance from the end of the casing assembly 20. In this space are assembled a plurality of guide vanes 45 which are held between the inwardly extending flange 44 and the lower end of the suction inlet casing assembly 20 by a plurality of circumferentially spaced bolts 46 so that the guide vanes 45 form a plurality of fluid flow paths or by-pass outlets as at 47 which communicate at one end with the lower end of the annular recirculating chamber or bypass passage 41 and at the end remote therefrom with the inlet opening 31 of the suction inlet upstream of the inducer 35.

The upper end of the annular recirculating chamber or bypass passage 41 communicates through a plurality of circumferentially disposed ports 48 in the wall of the casing assembly 20 with a point in the suctioninlet 30 downstream of the inducer and upstream of the impeller a.

Thus, when fluid is passing through the suction inlet it is free to pass both to the impeller 25a or through the bypass ports 48 into the recirculation chamber or bypass passage 41. In the recirculation chamber or bypass passage 41 it flows out through the bypass outlets 47 which return the recirculated or bypassed fluid to the inlet opening 31 for the suction inlet 30. The amount and quantity of fluid bypassed is a function of the differential pressure or head across the inducer as is more fully explained below. However, additionally as can be seen with reference to FIGS. 2 and 3 of the drawings the guide vanes which are spaced circumferentially about the longitudinal axis of the pump 10 are of streamlined shaped and are disposed with respect to each other so as to impart rotational flow to the bypassed or recirculated fluid passing through the plurality of fluid flow paths or bypass outlets 47 formed between the inwardly extending flange 44 and the lower end of the casing 20.

The direction the recirculated or bypassed fluid will be prerotated will be the same direction as the direction of rotation of the inducer for the reasons discussed below in more detail.

Since the operation of the pump does not differ from that above described the advantageous results obtainable from the bypassing and prerotation of the fluid being pumped by means of the above described structure is best understood with reference to the graphical illustrations at FIGS. 8-1 1 of the drawings.

i. e. that blade angle and curvature of the inducer blade I for the ,given velocity (RPMs of the driver) of the blade; such that the differential pressure or head across the inducer will approach zero as the capacity reaches the design condition, an empirical condition which varies from inducer to inducer.

As the differential pressure approaches zero the in ducer produces little head and requires very little horsepower to drive it.

The inlet diagram for the flow conditions at design or normal flow capacity for the centrifugal pump in which such inducer is utilized is illustrated at FIG. 9 of the drawings wherein:

C(md), represents the through flow velocity of fluid along the axial direction past the inducer blades,

it, represents the circumferential velocity of the blade elements of the inducer,

R, the relative velocity of the fluid flow past the inducer, and

a, the angle of the inducer vane to result in a very low attack angle between the axial flowing fluid and the blade angle of the inducer.

The inlet diagram for conditions at reduced flow C(mrf) is illustrated at FIG. 10 of the drawings in which it can be seen, that the relative flow is now at angle a, and the difference between a and a, is the increased angle ofattack between the fluid and the angles of the blades of the inducer. As a result there is increased differential pressure or a higher head across the inducer.

The increased head requires increased horsepower to drive the inducer which is provided by increasing the size of the d driver or prime mover operating the pump if. the pump is to be effectively operated at reduced flows for any great-length of time.

Additionally, as will be-understood by those skilled in the art the increased pressure will require higher rated valves, piping, etc. downstream of the pump.

In the present invention this problem is overcome by adding a radial component to the fluid passing the inducer of the pump. 1

This prerotation is effectively accomplished by taking advantage of rise in inducer head to provide an automatic means to bypass a portion of the fluid moving axially after it has passedthe inducer. This bypassed fluid is returned to the inlet end of the suction inlet or upstream side of the inducer and beforeit enters the suction inlet rotation is imparted thereto by means of the angled guide vanes. The degree of prerotation is such that the angle of attack at partial flows between the axial flowing fluid and the blades of the inducer is returned to that for the design conditions.

This prerotation and the degree of prerotation will be in the same direction of rotation as the inducer is rotated.

The inlet diagram for conditions at reduced flow v where fluid is bypassed and prerotated C(MBp) is illustrated at FIG. 1 l of the drawings.

By reference to FIGS. and 7 the connecting conduit 120, the inwardly extending flange 133 and the plurality of circumferentially spaced guide vanes 135 are connected to the end of the axially extending projection 117 by means of a plurality of circumferentially spaced bolts 136.

The respective'guide vanes 135 are shown as having a streamlined or tear dropped shape and in assembled position form a plurality of bypass outlet passages as at 137 between each of said guide vane members 135 which communicate at one end with the recirculation chamber or bypass passage 131 and at the other end with the inlet 119 for the suction inlet of the pump or at a point upstream of the inducer 127.

By reason of the shape of each respective guide vanes it is clear that they can be angled so that they will rotate fluid passing through the passages 137 in any desired direction and as illustrated in FIG. 7 the fluid passing through the passages 137 will be turned or prerotated in the same direction indicated by the inducer by the arrow shown in the drawings.

The end of the recirculation chamber or bypass passage 131 remote from the bypass outlets 137 communicates with the suction inlet 118 at a point downstream of the inducer 127 and upstream of impeller 125 through a plurality of circumferentially spaced ports 138 in the wall of the axially extending projection 117.

The operation of the recirculation or bypass means and the prerotation means is identical to that above described for the FIG. 14 form of the invention at normal flow.

At design conditions with little differential pressure I or head across the inducer very little fluid will be bypassed through the vrecirculation and bypass means above described. However, at reduced or partial flow by the same process of regulation suitable amounts of fluid will be bypassed through the recirculation and prerotation device to maintain the desired zero angle of attack between the flowing fluid and the blades of the inducer and thus achieve the advantageous result of this arrangement.

The proper dimensions for fluid openings, passages and paths associated with the bypass or recirculation arrangement as described in each form of the present invention as well as the desired angle of settings for the guide vanes for introduction of the desired degree of prerotation can all be determined empirically or by calculations as will be understood by those skilled in the art.

It will be further understood that the ideal design situation occurs when the amount of fluid being recirculated and the effect of prerotation for any reduced fluid flow condition is effective to totally negate the effect of such reduced fluid flow on the operating characteristics. While such ideal conditions are difficult to achieve, the provision of the recirculation means and the means for establishing prerotation of the recirculated fluid all in accordance with the present invention serves to improve the operating performance of any inducer in a centrifugal pump under operating conditions wherein the flow is less than the flow for which the inducer is designed.

While the foregoing has illustrated and described what is now contemplated to be the best mode of carrying out the invention the construction is, of course, subject to modification without departingfrom the spirit and scope thereof. Therefore, it is not desired to restrict the invention to the particular form of construction illustrated and described but to cover all modification that may fall within the scope of the appended claims.

What is claimed is:

1. A centrifugal pump including:

a suction inlet;

a discharge outlet;

at least one pumping stage having an impeller in direction communication with the suction inlet;

inducer means disposed in suction inlet operatively associated with said impeller;

means for rotating said impeller and said inducer means;

self regulating by-pass means having inlet means upstream of the inducer means and outlet means downstream of the inducer means for recirculating a given portion of the fluid passing through the suction inlet as a direct function of the differential pressure established across the inducer means by the fluid flowing through said suction inlet;

and guide vanes mounted in the bypass means and angled to impart prerotational motion to the recirculated fluid in the same direction of rotation as that of the inducer means to effect a reduction in head across the inducer means particularly at reduced flow conditions.

2; In a centrifugal pump as claimed in claim 1 wherein:

I the self regulating bypass means comprises an annular member connected to said pump about the suction inlet to form a bypass passage;

port .means in said suction inlet at a point downstream of said inducer means for delivering fluid to said bypass chamber;

means forming an outlet for said bypass passage communicating with the suction inlet upstream of said inducer means;

and said guide vanes disposed in the means forming said outlet means for said chamber.

3. In a centrifugal pump as claimed in claim 1 wherein:

a casing forms said suction inlet;

a member about said casing and connected in spaced relation thereto forms the passage means for recirculating the fluid in the self regulating bypass means.

4. In a centrifugal pump as claimed in claim 3 wherein port means are provided in the casing forming the suction inlet at a point downstream of said inducer to pass fluid to said passage means to be bypassed;

said annular member provided with a radially inward extending flange disposed a spaced distance from the end of the casing;

and said guide vanes connected between the end of said casing and said inwardly extending flange to provide a plurality of bypass outlet for passing bypassed fluid from said passage means to the suction inlet upstream of said inducer means.

5. In a centrifugal pump as claimed in claim 1 wherein:

means are provided to vary the angular position of the guide vanes for imparting the desired degree and direction of rotation to said bypass fluid returning to the suction inlet. 

1. A centrifugal pump including: a suction inlet; a discharge outlet; at least one pumping stage having an impeller in direction communication with the suction inlet; inducer means disposed in suction inlet operatively associated with said impeller; means for rotating said impeller and said inducer means; self regulating by-pass means having inlet means upstream of the inducer means and outlet means downstream of the inducer means for recirculating a given portion of the fluid passing through the suction inlet as a direct function of the differential pressure established across the inducer means by the fluid flowing through said suction inlet; and guide vanes mounted in the bypass means and angled to impart prerotational motion to the recirculated fluid in the same direction of rotation as that of the inducer means to effect a reduction in head across the inducer means particularly at reduced flow conditions.
 2. In a centrifugal pump as claimed in claim 1 wherein: the self regulating bypass means comprises an annular member connected to said pump about the suction inlet to form a bypass passage; port means in said suction inlet at a point downstream of said inducer means for delivering fluid to said bypass chamber; means forming an outlet for said bypass passage communicating with the suction inlet upstream of said inducer means; and said guide vanes disposed in the means forming said outlet means for said chamber.
 3. In a centrifugal pump as claimed in claim 1 wherein: a casing forms said suction inlet; a member about said casing and connected in spaced relation thereto forms the passage means for recirculating the fluid in the self regulating bypass means.
 4. In a centrifugal pump as claimed in claim 3 wherein port means are provided in the casing forming the suction inlet at a point downstream of said inducer to pass fluid to said passage means to be bypassed; said annular member provided with a radially inward extending flange disposed a spaced distance from the end of the casing; and said guide vanes connected between the end of said casing and said inwardly extending flange to provide a plurality of bypass outlet for passing bypassed fluid from said passage means to the suction inlet upstream of said inducer means.
 5. In a centrifugal pump as claimed in claim 1 wherein: means are provided to vary the angular position of the guide vanes for imparting the desired degree and direction of rotation to said bypass fluid returning to the suction inlet. 